The invention relates to a system and method for generating transmit clock timing and, more particularly, to a system and method for generating transmit clock timing based on receive clock timing in a communications system, such as a digital cordless telephone.
In electronic communications systems, timing of transmissions and receptions is important. In communications between units of those systems, the units must have sufficient transmission and reception timing capabilities to achieve desired communications. In many of those systems, certain synchronization of timing among communications units is necessary for accomplishing the communications. One particular application in which synchronization of timing is important is communications between a base unit and hand unit of a digital cordless telephone.
System timing synchronization may be achieved in a variety of ways. If the system is a time division multiplex system, then transmissions and receptions of communications between units of the system occur at different times over the same radio frequency. In such a system, synchronization of timing of units is necessary to ensure appropriate signal transmission and reception, without loss of transmitted and received data.
Though there are various ways to achieve synchronization in electronics communication systems, a typical way to synchronize two communicating units has been to make one of the units a timing slave to the other unit. In a digital cordless telephone, for example, a base unit may serve as a timing master and a hand unit may serve as a timing slave. In such an arrangement, the receive timing of the hand unit may be derived from the base unit transmission timing via a signal received by the hand unit from the base unit. The transmit timing of the hand unit may, then, be derived, for example, from the receive timing as is typical. In this manner, synchronization of timing among the base unit and hand unit is achieved.
A typical arrangement for achieving synchronization according to this typical synchronization scheme employs a phase lock loop in the slave unit, presumed herein to be the hand unit (for purposes of discussion only, and not intended to be limited to that case). The phase lock loop serves to lock on the signal received by the hand unit and to recover a timing signal (herein sometimes "clock") from that received signal. The received signal, itself, is then utilized by the hand unit to dictate the hand unit timing. In this typical configuration, transmit timing of the hand unit is derived directly from the receive timing, and the phase lock loop is employed to compensate for jitter of the received signal and to lock on that signal in order to derive transmit timing. When the received signal so dictates timing of the hand unit, the hand unit acts as a timing slave to the base unit, thereby resulting in synchronization.
This typical arrangement has certain disadvantages. In particular, because the transmit timing is derived directly from the received signal and phase lock loop operations, the jitter on the transmit timing is dictated by the worst jitter on the received signal. This can have particularly undesirable effects in instances, for example, of a bad channel over which the received signal is communicated to the hand unit by the base unit. In such an instance, the jitter from the bad channel is imprinted to the transmit timing because the transmit timing is derived from the received signal. Furthermore, the transmit timing derived from the received signal may be jumpy because of the adjustments made in the timing of the received signal via employment of the phase lock loop of the hand unit.
What is needed is a system that reduces jitter effects on the transmit timing, and that requires fewer adjustments to the timing to maintain synchronization.